Analogue Step Sequencer

Adding some beats to an analogue 'toy' synth ...

Here is an analogue step sequencer, of a type known as a Baby 8. The timer is provided by an NE555 IC (top right below) and the sequencer part by a CD4017 decade counter IC,  which has 10 outputs available, numbered 0 to 9. The sequencer uses outputs 0 to 8 only, by having the purple wire to the right of the chip connecting the 9th pin (output 8) to the reset pin (15). 

I played around with an LED chaser kit based on the same chips, but found it difficult to adapt using the supplied PCB, so used stripboard instead. It's rather basic and to keep costs right down I used circuit-mounted potentiometers that are adjustable by screwdriver. The corners of the stripboard were cut off so it would fit into a container, btw!

The header pins at the right perform the following functions, numbering them 1 to 5, top to bottom:

1. Power +ve: MUST NOT exceed the voltage rating of the sequenced instrument, otherwise you may damage it!

2. Power -ve

3. Ground. connected to Power -ve: to connect to instrument's ground

4. Control Voltage (CV) out: voltage for the instrument's Voltage Controlled Oscillator (VCO)

5. Clock output: for use as a gate signal: MUST NOT be added to the CV out, as it might blow the synth!

The instrument I used is shown below: a Gakken SX-150 Mark II handheld analogue synthesizer. The CV out from the sequencer is connected to the metal stylus and the ground from the sequencer to the outside of one of the sockets.

Each note in the sequence is tuned by a small 100k (104) ohm pot. Tuning using these is fiddly and is not helped by the value used: with a 5V power supply and the Gakken synth, 50k ohm pots would have spanned the full range and would have been a better choice.

Overall pitch can be adjusted by a 500k (504) ohm pot. This was my own 'innovation'. However, the value is way too big: 50k ohm for coarse and perhaps 10k ohm for fine adjustment would have been far better.

Clock speed can be finely adjusted by the 50k (503) ohm pot (as it was for the LED chaser kit) and coarsely by a 500k (504) one (again, my own 'innovation').

Some comments

• Connecting the gate out rather than  CV to the synth results in a single pitch percussive note

• Using all 8 steps and 5V, I get a low pitch tone playing between the notes. This is not (possibly less...) apparent when using a 3V supply which results in lower pitch notes, presumably making the interval tone inaudible. In this mode, notes can't be shaped using envelope attack and delay (eg. for a brassy or a plucked string sound).

• To separate the notes properly, and to shape them, I use half of them as pauses between 4 actual notes

• Great fun can be had, even with just 4 notes, by selecting a suitable tempo on the sequencer and twiddling with the knobs on the connected synth (e.g.  filter cutoff and resonance)

• Perhaps a good use would be for a repeated percussive or bass line rather than melody

Connecting to other instruments

• I tried connecting the sequencer to a Stylophone hand-held electronic organ, again attaching the CV to the stylus, but that didn't work as that has individually tuned keys rather than a resistor strip 'keyboard' as used by the Gakken.

• I have also tried this with a Korg Monotron. I took the synth apart and held the CV input from the sequencer to a point on the PCB, and IIRC, the clock/gate input from the sequencer to another point on the PCB. The ground connection from the sequencer went to the negative battery terminal on the synth. A permanent solution would be to solder wires to appropriate points on the Korg's PCB and then to use the other ends to make an interface between the sequencer and synth.

Improvements/Plans for a Mega 8 perhaps?

• Provide a pre-tuned octave of 13 notes (8 white and 5 black), plus a 'rest' note and a couple of switches to lower/raise notes by an octave

• Allow the user to define sequences of these notes using a patch panel - no need for continual tuning!

• Provide some stored sequences wired to these notes. Using header pins these could be made as modules to physically slot into place. Ideally they would be selected using switches, but each pin on the CD4017 IC would need to be switchable. Perhaps instead include a CD4017 with each stored sequence, so just the clock input needs to be switched, which may seem like overkill, but these ICs are as cheap as chips...

• Provide a 'keyboard' consisting of momentary switches connected to these notes

• Allow input to be selected from the 'keyboard', from the user-defined sequencer or from the stored sequences, using switches and highlight the choice made using LEDs. Switching needs to be exclusive - mustn't accidentally add input signals and risk damaging the synth.

• Connect/'cascade' two decade counters and allow up to say 16 notes in a sequence

• Some buttons to interrupt the sequencer would make it more interesting: perhaps one to restart the a sequence using pin 15 and another to hold the note being played using pin 13

• Add LEDs! Blinking lights are always fun and would show which note in the sequence is being played and could help match the required tempo in a performance.

While I thought the sequencer was a fun project and had potential, I didn't take it any further however, preferring the digital route using an Arduino, which I will cover elsewhere.

For completeness, here is the underside of the stripboard. IIRC, I used a drill bit with a makeshift handle in my hand to create the gaps in the copper strips.

References

https://oshwlab.com/dmitriy.prangov/Atari-Punk-Console-with-8-step-sequencer

http://www.dinsync.info/2010/05/diy-8-step-sequencer.html

https://hackaday.com/2016/01/14/oh-baby-baby10-build-a-classic-analog-music-sequencer/

(I don't care much for the APCs mentioned here and attaching a sequencer to it doesn't make it any better, IMO! If you don't have a synth to hand, perhaps use the sequencer with a simple DIY electronic organ instead?)